Preserved Wook is an oldtime New England railway engineman who calls to life again the history & technology not only of the steam locomotive in the LAST Steam Age of the Old Atlantic West, but of many a wonderful way of getting around in the World, in a better time when travel indeed was for the few…and the very few!

Mirror, Mirror on The Wall…

The debate about which was the “greatest,” the “most powerful” or even just the “biggest” of America’s late-modern articulated steam locomotives shows no sign of reaching a final conclusion. Nor can it be expected to do. The variables are simply too numerous. However, there is another aspect to this matter of what is after all a question of quality. On this last point, there indeed are other considerations besides those of mass and inertia and ox-like power. These in turn must bring us to those concerns with superior design that may even trump the comparatively crass marxist question of which American articulated engine hauled in its career the most total value in cargo. This final question however — a matter of engineering excellence — just perhaps is going to be at least a tad easier to resolve, and in fact I think I just might have found the answer, already. Here then is an account of what may well have been America’s finest articulated steam locomotive of them all in the last (NB) American Age of Steam:

The Norfolk & Western Railway’s 2-8-8-2 Y6b was the last in a long line of true Mallet compound expansion locomotives. Following E W King, Jr, writing in the pp of Drury [a], we read that N&W made a different decision early in its ongoing development of its line of compound heavy-duty coal haulers. Most railroads were going down the mainline of ever larger 2-8-8-2s, “lumbering beasts with immense pulling power, but little capacity for speeds over 20 mph.” [b]
What inhibited the N&W was the matter of close clearances along its right of way writes King, presumably especially in transmontane tunnels.
There simply wasn’t room to pursue infinitely the promiscuous deployment of larger and larger low-pressure cylinders to the point where, possibly, their combined diameter could even have come in principle to equal that of the locomotive boilers. (I believe I recall reading that the largest low-pressure cylinder diameter in North America approached 42″!)
Accordingly, N&W design engineers set out on the more refined and sufi-like path of attainment:

“[T]o obtain the desired power N&W used smaller cylinders and boiler pressure of 230 pounds — 200 pounds had been considered the maximum. This produced more efficiency and speed than was possible in contemporary designs, and in 1919 the WW I-era USRA [c] 2-8-8-2s…used an even higher boiler pressure — 240 pounds.
“The speed potential of the USRA 2-8-8-2 and its derivatives with higher pressure and smaller cylinders was [such that] it is a tribute to the USRA designers that no USRA or USRA-derived Mallet [compound] was ever converted to [single expansion.]
“Alone among America’s railroads, N&W [designers and managers saw] that the compound 2-8-8-2 based on the USRA design could be modified to minimize back-pressure problems and “souped up” to be an ideal mountain heavy freight hauler. In tinkering with the design over a period of 33 years the road wound up with [the Y6b 2-8-8-2] capable of producing 5600 drawbar horsepower at 25 mph with a top speed of 50 mph — perfect matches for N&W’s tonnage, grades and curves — and all this was achieved while retaining the economies of compound operation and in a locomotive that weighed 100,000 pounds less [!] than either Chesapeake & Ohio’s 2-6-6-6 or Union Pacific’s 4-8-8-4. [d]

Y6b with specs, University Libraries digital collection

The Y6b class did not begin to emerge from the Roanoke shops until 1948. Numbers 2171-2187 were based on earlier developments, especially in the Y6 series and now utilized Worthington feedwater heaters. “Roanoke turned out Y6bs 2188-2200 between August 1950 and April 1952; 2200 was the last steam locomotive built for road freight service in the United States.” These last locomotives featured roller bearings all around. Cylinder-valve improvements in the last five Y6bs “increased the power of the low-pressure engines [e] and required the addition of 29,000 pounds of lead to them (the engines still weighed 110,000 pounds less than a Union Pacific 4-8-8-4). [A booster valve] increased the power of the low-pressure engine in compound operation by admitting a small amount of steam at boiler pressure [now at 300 pound psi — PW] into the receiver…. These Mallets now could develop 170,000 pounds tractive effort simple, 132,000 pounds compound, and produce 5600 drawbar horsepower at 25 mph [as already noted.] The last N&W Y6b ran in revenue service in May 1960…a tribute to the continuing intelligent modification [over two generations — PW] of a basically sound design.” [f]